Dihydropyran is an important core structure as often found in bioactive natural products, pharmaceuticals and intermediates thereof. These dihydropyran compounds can be obtained by hetero Diels-Alder reaction. For example, the hetero Diels-Alder reaction with inverse electron demand, catalyzed by CrIII-Schiff base complexes is reported to synthesize 3,4-dihydro-2H-pyran compounds from vinyl ether compounds and α,β-unsaturated aldehydes (see Non-Patent Literature 1). One-pot formation of chiral polysubstituted 3,4-dihydropyrans via a novel organocatalytic domino sequence involving alkynal self-condensation is also reported (see Non-Patent Literature 2).